[flang] Allow module, submodule, and program names to be used as local identifiers. Check for USE of m in m.

Original-commit: flang-compiler/f18@9abfd9e450
Reviewed-on: https://github.com/flang-compiler/f18/pull/737
Tree-same-pre-rewrite: false

flang/lib/semantics/resolve-names.cc

@@ -1653,7 +1653,11 @@ void ScopeHandler::PushScope(Scope &scope) {
   if (kind != Scope::Kind::Block) {
     ImplicitRulesVisitor::BeginScope(scope);
   }
-  if (!currScope_->IsDerivedType()) {
+  // The name of a module or submodule cannot be "used" in its scope,
+  // as we read 19.3.1(2), so we allow the name to be used as a local
+  // identifier in the module or submodule too.  Same with programs.
+  if (!currScope_->IsDerivedType() && kind != Scope::Kind::Module &&
+      kind != Scope::Kind::MainProgram) {
     if (auto *symbol{scope.symbol()}) {
       // Create a dummy symbol so we can't create another one with the same
       // name. It might already be there if we previously pushed the scope.
@@ -2086,7 +2090,7 @@ void ModuleVisitor::BeginModule(const parser::Name &name, bool isSubmodule) {
 // If an error occurs, report it and return nullptr.
 Scope *ModuleVisitor::FindModule(const parser::Name &name, Scope *ancestor) {
   ModFileReader reader{context()};
-  auto *scope{reader.Read(name.source, ancestor)};
+  Scope *scope{reader.Read(name.source, ancestor)};
   if (!scope) {
     return nullptr;
   }
@@ -2094,6 +2098,9 @@ Scope *ModuleVisitor::FindModule(const parser::Name &name, Scope *ancestor) {
     Say(name, "'%s' is not a module"_err_en_US);
     return nullptr;
   }
+  if (DoesScopeContain(scope, currScope())) {
+    Say(name, "Module '%s' cannot USE itself."_err_en_US);
+  }
   Resolve(name, scope->symbol());
   return scope;
 }

flang/test/semantics/resolve05.f90

@@ -13,9 +13,24 @@
 ! limitations under the License.
 
 program p
-  !ERROR: 'p' is already declared in this scoping unit
-  integer p
+  integer :: p ! this is ok
 end
+module m
+  integer :: m ! this is ok
+end
+submodule(m) sm
+  integer :: sm ! this is ok
+end
+module m2
+  type :: t
+  end type
+  interface
+    subroutine s
+      !ERROR: Module 'm2' cannot USE itself.
+      use m2, only: t
+    end subroutine
+  end interface
+end module
 subroutine s
   !ERROR: 's' is already declared in this scoping unit
   integer :: s

flang/test/semantics/resolve20.f90

@@ -48,6 +48,9 @@ module m
   type :: bad3
   end type
 
+  type :: m ! the name of a module can be used as a local identifier
+  end type m
+
   external :: a, b, c, d
   !ERROR: EXTERNAL attribute not allowed on 'm'
   external :: m